Signal input circuits are known from Siemens "Semiconductor Circuitry Examples, Single Semiconductors and Integrated Circuits", 1972/73, pages 38 to 41. In the known circuitry example, there is connected in the signal path from the antenna input to the preamplifying component a control network in .pi. circuit arrangement with PIN diodes acting as controllable resistors. For regulating (controlling) the signal transmission (amplification), the PIN diode in the longitudinal branch of such an arrangement is controlled by means of the forward currents from a control signal source inversely to the PIN diodes in the transverse branches. This results in a HF voltage division in the direction of the amplifier component, caused by the inversely changng resistances of the PIN diode in the longitudinal branch and the PIN diode in the transverse branch which faces the amplifier component. With the PIN diode of the other transverse branch, the load change occurring to the longitudinal branch is at least partly balanced.
This prevents unwanted strong signal reflection when used in a television receiver. Owing to the passive character of such a control network, only the transmission loss and, furthermore, the amplification of the input stage can be adjusted, i.e., controlled. The advantage of amplification control of an input stage by means of PIN diodes in the manner described is that during large-signal operation, the following non-linear components such as transistors and tuning diodes can be protected from driving which causes distortions. However, the known arrangement necessitates an elaborate component system and high controlling power requirement.